Not Just a Plasma Membrane Protein: in Cardiac Muscle Cells Alpha-II Spectrin also Shows a Close Association with Myofibrils
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Spectrin and its associated proteins are essential for the integrity of muscle cells and there is increasing evidence for their involvement in signalling pathways as well as having a structural function in mediating stress. Spectrin is a multigene family and it is essential to determine which isoforms are present and their location in the cell. In heart muscle, we have found that one spectrin isoform, αII-spectrin, is strongly represented and, using immunofluorescence, we show that it lies within the contractile fibres near the Z-disc as well as on the cardiomyocyte plasma membrane. Electron microscopy of immunogold-labelled cryosections reveals statistically significant clustering of gold particles near the Z-disc, within and close to the edge of myofibrils. βII-spectrin and ankyrin-R and G are both known to occupy this region. We suggest that αIIβII spectrin tetramers with ankyrin organise and/or stabilise cardiac muscle cell membrane components relative to the contractile apparatus.
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